Adapted from Bon Appétit
4 ounces semi-sweet chocolate, chopped
3 cups cake flour
1½ tsp. baking powder
½ tsp. salt
3 large eggs
2 cups sugar
1 cup vegetable oil
1 cup whole milk
1 tsp. vanilla extract
4 tsp. ground cinnamon
1/3 cup whipping cream
¼ cup (½ stick) butter
2 tbsp. light corn syrup
6 ounces semisweet chocolate, chopped
Position rack in center of oven and preheat to 325ºF. Grease a 12 cup bundt pan and set aside. Place chocolate in a medium microwave-safe bowl. Heat in 30-second intervals, stirring in between each one, until the chocolate is almost melted. Allow the residual heat to melt the rest of the chocolate and set aside. (Alternatively, you can set the bowl over simmer water. Do not allow the bottom of the bowl to touch the water.)
Sift flour, baking powder and salt into medium bowl. Using electric mixer, beat eggs in large bowl until foamy. Add sugar and beat until thick and fluffy, about 3 minutes. Gradually beat in oil. Beat in milk and vanilla. Add dry ingredients and beat until just blended.
Transfer 1½ cups batter to bowl with the melted chocolate. Stir to combine. Mix cinnamon into remaining (non-chocolate) batter. Spread half of cinnamon batter in prepared pan. Spoon chocolate batter over. Top with the remaining cinnamon batter. Using small knife, swirl batters together to marbleize slightly.
Bake cake until tester inserted near center comes out clean, about 1 hour to 1 hour 10 minutes. Cool cake in pan on rack 10 minutes. Using a thin knife, carefully loosen the cake from the pan. Turn cake out onto rack and allow to cool completely.
Combine cream, butter, and corn syrup in heavy medium saucepan. Stir over medium heat until mixture simmers. Remove from heat; add chocolate. Stir until chocolate melts and glaze is smooth. Let cool until glaze thickens slightly but is still spreadable, stirring occasionally, about 30 minutes.
Place cake on platter. Slide waxed paper strips under edges of cake (this will catch drips). Spread glaze over cake. Remove paper. Chill until glaze sets. Serve at room temperature. (Can be made 2 days ahead. Cover and keep chilled. DT: In my experience, you do not need to chill this cake – it keeps well at room temperature, but be sure to cover it. Also, cakes like this freeze beautifully – without the glaze. Just wrap it well in foil and it will keep for about a month. Give it plenty of time to thaw at room temperature and then glaze it.)